01 -
Heat the oven to 400°F and move the racks to the middle and top spots. Cover a baking sheet with parchment paper.
02 -
Melt butter and cook sliced onions with thyme and seasonings for about half an hour, stirring now and then, until rich golden brown. Sprinkle in flour, pour in wine, and cook until the wine is gone.
03 -
In a bowl, stir together thyme, garlic, grated cheese, breadcrumbs, milk, egg, Worcestershire, salt, and pepper. Gently mix in the ground beef. Shape 32-34 small meatballs.
04 -
Put the meatballs on the parchment-lined sheet and bake for 15-20 minutes, or until their centers reach 165°F.
05 -
Pour in the broth and let it simmer until it gets thick. Mix in balsamic or sherry vinegar, then adjust the taste as needed.
06 -
Add the meatballs into the sauce, sprinkle cheese on top, and broil for 3-4 minutes so the cheese gets bubbly. Finish with a little thyme on top for garnish.